madvise man page on CentOS

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MADVISE(2)		   Linux Programmer's Manual		    MADVISE(2)

NAME
       madvise - give advice about use of memory

SYNOPSIS
       #include <sys/mman.h>

       int madvise(void *start, size_t length, int advice);

DESCRIPTION
       The madvise() system call advises the kernel about how to handle paging
       input/output in the address range beginning at address start  and  with
       size  length  bytes. It allows an application to tell the kernel how it
       expects to use some mapped or shared memory areas, so that  the	kernel
       can  choose  appropriate	 read-ahead and caching techniques.  This call
       does not influence the semantics of the application (except in the case
       of  MADV_DONTNEED),  but	 may  influence its performance. The kernel is
       free to ignore the advice.

       The advice is indicated in the advice parameter which can be

       MADV_NORMAL
	      No special treatment. This is the default.

       MADV_RANDOM
	      Expect page references in random order.  (Hence, read ahead  may
	      be less useful than normally.)

       MADV_SEQUENTIAL
	      Expect  page  references	in sequential order.  (Hence, pages in
	      the given range can be aggressively read ahead, and may be freed
	      soon after they are accessed.)

       MADV_WILLNEED
	      Expect  access  in  the near future.  (Hence, it might be a good
	      idea to read some pages ahead.)

       MADV_DONTNEED
	      Do not expect access in the near future.	(For the  time	being,
	      the  application is finished with the given range, so the kernel
	      can free resources associated with it.)  Subsequent accesses  of
	      pages  in this range will succeed, but will result either in re-
	      loading of the memory contents from the underlying  mapped  file
	      (see  mmap())  or zero-fill-on-demand pages for mappings without
	      an underlying file.

RETURN VALUE
       On success madvise() returns zero. On error, it returns -1 and errno is
       set appropriately.

ERRORS
       EAGAIN A kernel resource was temporarily unavailable.

       EBADF  The map exists, but the area maps something that isn't a file.

       EINVAL The  value len is negative, start is not page-aligned, advice is
	      not a valid value, or the application is attempting  to  release
	      locked or shared pages (with MADV_DONTNEED).

       EIO    (for  MADV_WILLNEED)  Paging  in	this  area  would  exceed  the
	      process's maximum resident set size.

       ENOMEM (for MADV_WILLNEED) Not enough memory: paging in failed.

       ENOMEM Addresses in the specified range are not	currently  mapped,  or
	      are outside the address space of the process.

LINUX NOTES
       The current Linux implementation (2.4.0) views this system call more as
       a command than as advice and hence may return an error when  it	cannot
       do what it usually would do in response to this advice. (See the ERRORS
       description above.)  This is nonstandard behaviour.

       The Linux implementation requires  that	the  address  start  be	 page-
       aligned,	 and  allows length to be zero. If there are some parts of the
       specified address range that are not mapped, the Linux version of  mad‐
       vise()  ignores	them  and  applies  the	 call to the rest (but returns
       ENOMEM from the system call, as it should).

HISTORY
       The madvise() function first appeared in 4.4BSD.

CONFORMING TO
       POSIX.1b.   POSIX.1-2001	 describes  posix_madvise()   with   constants
       POSIX_MADV_NORMAL, etc., with a behaviour close to that described here.
       There is a similar posix_fadvise() for file access.

SEE ALSO
       getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2)

Linux 2.4.5			  2001-06-10			    MADVISE(2)
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